1. Field of the Invention
The present invention relates to a wireless terminal device, and particularly, a wireless terminal device provided with a direct conversion receiving circuit using an orthogonal mixer of a differential type.
2. Description of the Background Art
At present, wireless terminal devices such as a cellular telephone primarily employ heterodyne receiving circuits. Although the heterodyne system requires an intermediate frequency circuit, it can prevent leak of a local oscillation signal to an antenna side because the local oscillation frequency is different from the receive frequency, and therefore can prevent deterioration of sensitivity.
In contrast to this, a direct conversion (homodyne) system does not require an intermediate frequency circuit, but the local oscillation signal leaks to the antenna side, and therefore the sensitivity deteriorates because the local oscillation frequency is equal to the receiving frequency.
In the heterodyne system, the intermediate frequency circuit can hardly remove channels (which will also be referred to as “interference waves” hereinafter) other then a desired channel. In the direct conversion system, however, the interference waves are hardly attenuated, and are supplied to a base band circuit. Therefore, the base band circuit is required to have extremely high anti-interference-wave characteristics. However, for increasing the anti-interference-wave characteristics, it is generally required to increase a current flowing through the base band circuit. Therefore, the anti-interference-wave characteristics of the receiving circuit can be improved only to a limited extent.
In general, cellular telephones are strongly required to have small sizes, light weight and low power consumption. Therefore, it is desired to reduce the number of components of the cellular telephone.
Japanese Patent Laying-Open No. 10-224249 has disclosed in FIG. 5 a direct conversion receiver having a low-pass filer, which selects a signal of only a desired channel from a base band signal sent from an orthogonal mixer while suppressing a signal of a neighboring channel. However, this low-pass filter is not specifically disclosed.
Japanese Patent Laying-Open Nos. 10-22860 and 10-32516 have disclosed low-pass filters similar to the foregoing low-pass filter, but have not disclosed specific structures and operations.